What is a flash fire?
A flash fire occurs when a flammable cloud is ignited after it has dispersed but before it has diluted below its lower flammable limit, in conditions where there is not enough confinement or congestion to generate significant overpressure. The cloud burns back through itself in a few seconds, producing intense but very brief thermal exposure.
How flash fire harm is assessed
Because the event lasts only seconds, radiation to receptors outside the cloud is usually not the governing effect. Standard practice is to treat anyone inside the flammable envelope as a fatality and anyone outside it as unharmed. The flammable envelope is normally taken as the LFL contour, or the half-LFL contour where an allowance for concentration fluctuation is required.
This makes flash fire consequence entirely dependent on the dispersion calculation — the fire model adds little beyond deciding where the cloud edge is.
Flash fire versus vapour cloud explosion
The same cloud can produce either outcome. If it drifts into a congested or confined region — pipe racks, module decks, compressor houses — flame acceleration generates overpressure and the event is a vapour cloud explosion. In open, uncongested areas the flame stays subsonic and the event remains a flash fire.
Event trees in a QRA therefore branch on ignition timing and on whether the ignited portion of the cloud sits in congestion.
What drives the envelope size
- Release rate and duration from the source-term calculation.
- Gas density behaviour — a dense cloud travels much further at ground level.
- Atmospheric stability and wind speed, with stable low-wind cases giving the largest envelopes.
- Terrain and obstacles that hold the cloud or channel it.
Frequently asked questions
How long does a flash fire last?
Typically a few seconds — the time for the flame front to burn back through the cloud. That is why exposure is treated as binary inside or outside the envelope rather than through a dose calculation.
Can a flash fire escalate?
It can ignite secondary fires wherever it passes over leaking or spilled fuel, and it can ignite the source itself, converting the event into a sustained jet or pool fire.
Is the LFL or half-LFL contour used?
Both are used. LFL represents the mean flammable extent; half-LFL is a common conservative allowance for concentration fluctuations. The choice should be stated in the study basis because it materially changes the reported envelope.
Calculators for this topic
- Gas Dispersion — Atmospheric gas dispersion calculator
- Gas Discharge — Gas discharge rate calculator for pressurised releases
- Gas Detection Mapping — Gas detection coverage mapping